Influence of different electron beam surface pre‐treatments on the results of subsequent boriding‐part I

Boriding produce thick hard layers on cast iron components, which can improve their wear and corrosion behaviour. However, this potential cannot be fully exploited by a simple boriding due to the material specific presence of graphite. In that context, paper presents results two fundamentally different electron beam liquid surface treatments (remelting, cladding with nickel-based additive) possibilities limitations regarding subsequent boriding. The behaviour under conventional high temperatures (760 °C–860 °C), experiments low-temperature (600 °C–700 °C) were investigated. Under identical treatment conditions, compound layer thicknesses generated unalloyed surfaces (remelting) approx. 50 %–75 % greater than those alloyed (cladding). A two-layered boride structure generated, though phase compositions. Nevertheless, hardness all borided comparable. Surface measurements revealed supporting effect substrates plays decisive role up thickness 57 μm. layer-thickness range, is higher substrates. This knowledge should prove for selection composites corrosive and/or tribologically stressed components. Beim Borieren können dicke, harte Schichten auf Gusseisenbauteilen generiert werden, die deren Verschleiß- und Korrosionsverhalten verbessern können. Dieses Potenzial kann jedoch durch ein einfaches aufgrund des werkstoffspezifischen Vorhandenseins von Graphit nicht vollständig ausgeschöpft werden. Desbezüglich werden in diesem Beitrag Ergebnisse zwei grundlegend unterschiedlichen Elektronenstrahl-Flüssigphasen-Randschichtbehandlungen (Umschmelzen, Auftragen mit Nickel-Basis-Zusatzstoff) Möglichkeiten Grenzen beim anschließenden vorgestellt. Es wurde das Borierverhalten bei konventionellen hohen Temperaturen Experimente zum Tieftemperaturborieren untersucht. Unter identischen Behandlungsbedingungen waren den unlegierten Oberflächen (Umschmelzen) erzeugten Verbindungsschichtdicken ca. dicker als der legierten (Auftragen). entstand eine zweiphasige Boridschicht, allerdings unterschiedlicher Phasenzusammensetzung. Dennoch Härtewerte aller Boridschichten vergleichbar. Oberflächenhärtemessungen ergaben, dass Stützwirkung Substrate bis zu einer Boridschichtdicke μm entscheidende Rolle spielt. Schichtdickenbereich ist Verbundhärte höher Substrate. Diese Erkenntnisse sind entscheidend für Auswahl Schichtverbunden korrosiv und/oder tribologisch beanspruchte Bauteile. wear, oxidation resistance highly components typically achieved means use expensive materials complex alloy designs (e.g., high-alloy steels, nickel base alloys, metal matrix composites, austempered ductile iron) 1. becoming increasingly important since, one hand, complex, thin-walled close final contour produced much more cost-effectively. On other iron's lower density (7.1 g/cm3) compared steel (7.8 lightweight construction concepts implemented. industrial application thwarted limited tribological load irons, improved either volume heat quenching tempering) or single treatments, as case steels nitriding, coatings) – only part 2-5. reason material-specific soft graphite, remains unaffected mentioned 6, 7. leads defects and, general, limitation local load-bearing capacity. For combination processes consisting thermal (laser beam) thermochemical well coatings have already established themselves advantageous technologies number applications, demand such solutions growing constantly 8-13. first scientific are also available iron, but these at advent (electron remelting+nitriding) not yet exhibited desired remelting+hard coating physical vapor deposition). What emerges from investigations, however, satisfactory liquid-phase remelting, alloying, cladding) resulting resolution graphite 8. solidification rates typical liquid-state lead well-known microstructural characteristics associated metastable solidification, fine-grained microstructures, supersaturation, phases intermetallic when using additives, alloying process), etc., an increase (improved capacity) 14-16. distinguished above significantly improving ability. Among reasons, composition (FeB, Fe2B) layers. commercial performed considerably (TB > 800 °C…<1150 places demands substrate 17. upper temperature limit respective chemical ( = f(wt.% carbon, silicon, sulphur, phosphor)) depends melts lowest temperature. Typical times range 1 h 8 time mainly influences fracture toughness decreases (4.54 MPa√m→2.19 MPa√m) increasing (3 h→8 h) formation FeB 18. Due anisotropy boron diffusion coefficient, anisotropic growth borides occurs, pronounced tooth-like interlocking both interface between 19, 20. Alloying elements reduce tips, new declines importance lateral favoured. As result, transition areas levelled 19. reality, relevant formed mixed crystals type (Fe, M)B M)2B, atoms substituted corresponding soluble chromium, manganese, nickel, cobalt, molybdenum others. Elements atomic Z (Z < 26), chromium 24), often boron-rich boride. contrast, element 28), favoured incorporation into M)2B enriched 21. result levelling interface, 22. copper insoluble pushed ahead front, so maximum concentration below layer, microstructure area 17, 23. When alloys agents, competing reactions occur: borating siliconizing 24. latter porous, granular silicides, Ni3Si, NiSi 24 Ni5Si2 25. Borides silicides unequal coefficients expansion, cracking boundary. Studies iron-nickel model show content Part Fe borides. contents Ni)2B correspond substrate, i.e., preferentially stored Ni)B lower. Ni)B. inclusion has hardly any influence Nickel enrichment occurs substrate. If too high, nickel-iron forms impairment best avoided 26. present contribution focuses comparative investigations (without additive (cladding)) spheroidal (EN-GJS-600) itself. circumstance common was had been almost completely removed very finely precipitated treatment. While remelted material, it changed during cladding. aim work was, analyse various microstructures test procedure's technological limits, particular regard reduction Based EN-GJS-600 (sample no. S600) pearlitic states set (EB) remelting (EBR) (EBC), Table main difference layer. additive, wire used, deposition generate rather surface. why we speak context. Electron defined process (wire) small melted 28. highest possible top Treatment sample Element [wt.%] C Si P S Cr Ni B Cu Mn Mo C45-Ref. Base 0.47 0.25 0.011 0.012 0.70 S600 3.5 2.3 0.024 0.008 0.6 0.34 EBR 3.4 2.0 0.021 0.006 0.5 0.26 EBC-1L 50.9 1.9 2.5 0.027 0.001 5.8 34.7 1.2 1.5 0.30 1.1 EBC-2L 23.5 1.0 2.7 0.029 0.009 10.1 56.6 2.1 0.20 1.7 NiCrBSi* 3.8 15.0 Bas. 3.15 3.0 addition, (C45-Ref.) comparable content, included behaviour, carried out universal chamber facility pro-beam systems GmbH, 2.6 m3 power 15 kW, providing acceleration voltage 80 kV. pre-heating TPH 500 °C before EB essential generating crack-free realised annealing field size samples (width × length: 60×100 mm2) in-situ pyrometric measurement. meander technique 27. special movement resulted combined motion energy spot, superimposed circular oscillation figure, x direction (vx) y direction, 2. track dimensions W×L: 20×100 mm2. Sample Beam current IB [mA] Frequency fy [Hz] Feed rate vX [mm/s] Wire feed vW [m/min] Oscillation figure [mm2] Mean thickness* [mm] 11 5 0,5 Circle: 0.75×0.75 0.8 38 3700 2 Ellipse: 1.5×2.5 5.5 flat cladded (EBC) adjacent overlapping (2 mm) tracks, applied alternately front rear feeding constant vW, With diameter 1.6 mm, positioned middle spot. interaction spot enlarged elliptical Both Lx determined content. selected preliminary tests. These showed total increased (1.4 mm 2.8 (24 mA 42 mA), while height decrease slightly (0.93 0.73 mm). approximately same melting (determined feed), input depth mixing expressed rising dilution 37 70 %. calculated ratio (below initial surface) (sum (above substrate). single-track terms (500 HV1) crack avoidance mA, kept thereafter. within effectively varied (EBC-1L: 35 wt.% nickel; 6 chromium) double (EBC-2L: 10 chromium). order level deformations, treated machined original parameters. Subsequently, pack-borided (BorTec GmbH & Co. KG) 600 °C, 700 760 820 860 intervals h, 7 agent developed was: 60 CaB6; B4C; KBF4. Macroscopic (SH) (HV1, HV0.05) (S600+B) duplex (EBx+B) served, among things, indicators effects load-supporting capacity cut metallographic cross sections prepared standard grinding polishing procedures. basis sections, characterised measured light optical scanning microscopy (SEM). measurement described chapter 3.3 microscopical images. Two coloured types could identified via backscattered contrast microscopy, thus facilitating (I, II) min/max values. hardness-depth profiles (HV0.3) (HV0.05) samples. near-surface profiles, average value characteristic (LHmax) mean analysed glow discharge emission spectroscopy. It noted there no calibration analysis systems, findings qualitative course obtained, quantitative assignment identification ≤10 X-ray diffraction Co-Kα radiation. deals (base material+boriding) treatment+boriding). addition (S600), C45 carbon incorporated facilitate silicon spectroscopy over μm–80 states, e.g. after mechanical processing. values given area, differed marginally compositions (wire: NiCrBSi). applied, provided especially comparison state dissolved Figure 1a. process-specific rapid rate, melt pool solidified according system iron-cementite. cementite factor 3 4, 1e. composed primary austenite dendrites, ultimately decomposed pearlite (P) cooling gradient pre-heating. embedded eutectic features ledeburite, coarse plates (C) rod-like eutectic, Figures 1b, 2a. Microstructure (a-d) (e, f) a) casting S600, b) EBR, c) single-layer d), double-layer EBC-2L. Gefüge (a–d) Härteprofile Ausgangszustände, Gießen EB-Umschmelzen einlagigem EBC-1L, d) doppellagigem constituents, morphologies, 1c, d. Furthermore, clear differences individual ratios therefore, rates. created regimes, namely (overlapped areas, II. exhibiting coarser microstructure, d (I). as-cast 250 HV0.3 420 (EBC-1L), 650 HV0.3–700 (EBC-2L) 1e, f. layers, caused fluctuations. whereas Hardness parallel z longitudinal section zones somewhat harder (490 HV0.3±38 HV0.3) molten (450 HV0.3±49 HV0.3), II). Using defraction (Co-Kα irradiation), ferrite (or state) detectable states. consisted predominantly austenite. led precipitation typified Fe3(Si, B) Ni31Si12, 2b. percentage detected microstructure. XRD-diffractograms states: remelted, single- (EBC-1L) (EBC-2L). XRD Diffraktogramme Ausgangszustände: wie gegossen umgeschmolzen, einlagig- zweilagig aufgetragen Despite ferritic/pearlitic reference conditions (700 °C/7 h), two-layer underlying Fe2B fashion similar 3a. connection materials. concentration-depth reflected Unetched SEM images (BSE modus) (860 substrates: (a) (variant A), (b) (example variant B). Ungeätzte REM-Aufnahmen (BSE-Modus) Boridschichtgefüge Substrate: (Variante (Beispiel Variante Characterisation example presented 3a depths tI tII. At low comparable, 4a. favour were, average, 20 thicker deviation significantly, sufficiently thick, remained f(tI), because Fe2B, Influence TB on: tII (determination 3a) profiles. Einfluss Boriertemperatur Substrates auf: Oberflächenhärte Gesamtschichtdicke (Bestimmung siehe Bild Härte-Tiefen-Profil. seen substrate-dependent microhardness 4b. concluded ΔHV 400 HV1, thickness, Conventional °C–880 40 μm–100 tB 5b (II). two-phased (FeB; Fe2B). temperature, increases (11 μm→53 μm), changes (32 →48 temperature-time regime boriding: (conventional) I. (tI) (tII) SH (HV1; FN 9.8 N) SHmax (HV0.05: 0.49 N), curves (S600+B). Temperatur-Zeit-Regimes während Borierens: niedrige Temperatur hohe (konventionell) Oberflächen- Schichtausbildung sowie Schichtdicke 9,8 maximale Härte im oberflächennahen Bereich (HV0,05: 0,49 bestimmt anhand Härte-Tiefen-Kurven (1052 HV1±28 (SHmax) (1341 HV0.05±47 relatively level, followed t≥40 ‘self-supporting’ F∼10 N. played subordinate role. Experiments (TB≤700 served determine limit, treatment, 5a. °C/24 single-phase (Fe2B) μm, hardness, supported corresponded 5a condition boronizing lie economic frame tB≤24 successfully °C. Investigations time-dependent mechanism form borides, After μm-thick two-phase formed, grew time, Interestingly, phase, steel, although, shown above, otherwise conditions. Pore evident materials: apparent near other, layer/substrate transition, pores initially disordered (16 %) 29, 30. directional disappeared, short (tB far-reaching redistributions led, complete decarburisation (cmin≈0.22 wt.%) 30 reached 638 HV1 half attainable insufficient. 5b. preceding (Chapter 3.1). Initial temperatures. Given thermally stable further development technology derived 27, 31. (phase composition) boriding, 1, (i.e., layer). investigated each exemplary variant: Variant A: state), B: (cladded states), 3b. A) macroscopically quite material. tooth-shaped structure, profile profile, 3a, 6a. secondary decomposition ledeburitic carbides stress nodular distributed homogeneously by, diffusion–especially graphite-forming silicon–and splash-like form, maxima (cx, max) elements, There element-specific position metallographically tmax (tI; tII), drawn blue dotted lines orientation, GDOES